Polyorganosiloxanes carrying organofunctional groups find wide use as organic resin modifiers. In particular, Japanese Patent Application Laid Open Number Hei 3-79626 (79,626/1991) and Japanese Patent Application Laid Open Number Hei 4-36321 (36,321/1992) teach that organofunctional group-endblocked polyorganosiloxanes can be copolymerized into polycondensation-type organic resins such as polycarbonate resins and polyimide resins and are extremely useful for modifying the functional properties of such resins. Unfortunately, the organofunctional polyorganosiloxanes used for this application have generally suffered from a low purity since they were prepared by an equilibration polymerization reaction. More specifically, these polyorganosiloxanes contained undesirable admixtures such as high molecular weight polyorganosiloxane and the organofunctional group-free organocyclosiloxane employed as starting material. The use of such a polyorganosiloxane mixture as an organic resin modifier makes it quite difficult to adjust or control molecular weight and prevents the production of organic resin with a constant or specified composition. The resulting organic resin will, in addition, suffer from impaired mechanical properties. The presence of the high molecular weight polyorganosiloxane also impairs heat resistance of the modified organic resin and leads to molding defects due to the conversion of a portion of the siloxane component to decomposition gases during the thermal processing that occurs when the resin is mixed or molded. Other problems have arisen from the admixture of the organofunctional group-free organocyclosiloxane starting material into the organic resin, such as (i) the occurrence of appearance defects in moldings prepared from the organic resin and (ii) the failure to obtain the desired properties due to elimination of the organocyclosiloxane during purification of the organic resin and the corresponding decline in the amount of polyorganosiloxane blended in the organic resin.